This invention relates to transducer arrays for ultrasonic diagnostic imaging systems and, in particular, to a two dimensional array which can be selectively operated for either two dimensional imaging or three dimensional imaging.
Both one dimensional (1D) and two dimensional (2D) transducer arrays are in use today for ultrasonic imaging. A 1D array consists of a flat row of transducer elements. A 1D array may be configured as a straight line of transducer elements or as a curved row of elements in the azimuth direction, which is the in-plane direction orthogonal to the beam directions which extend in the plane of the image in the range direction. The single row of elements may be controlled by selectively applying pulses at predetermined times to the individual elements to transmit a beam of ultrasonic energy which can be steered and focused in azimuth. The array can receive echoes from along the same beam direction. A single row of elements is confined to transmitting and receiving in a planar region in front of the emitting surface of the array. The 1D array has a fixed focus in the elevation dimension orthogonal to the image plane, which may be provided by an acoustical lens, curvature of the transducer, or both. This fixed elevational focus determines the thickness of the slice represented by the two dimensional image.
A 2D array is an array of transducer elements which extends in two dimensions, sometimes referred to as the azimuth and elevation directions, where the elevation direction is transverse to the azimuth direction. The 2D array is controlled in the same manner as the 1D array, by pulsing individual elements at selected times to transmit beams which can be steered and focused in both azimuth and elevation. 2D arrays can be either annular (composed of ring-shaped elements) or rectilinear (composed of rows and columns or other patterns of individual elements). Annular arrays formed of continuous rings can be focused in both azimuth and elevation but can only be directed straight ahead. Rectilinear 2D arrays can be focused and steered in both dimensions and hence can be used to steer beams through a three dimensional volumetric regions for three dimensional imaging.
Other, more restricted, variation of the 2D array are known, referred to as 1.5D and 1.75D arrays. A 1.5D array generally has fewer elements in the elevation direction than in the azimuth direction, and has pairs of elements symmetrically located on either side of a central row of elements. This enables the 1.5D array to be dynamically focused in the elevation direction, but the symmetrical operation of the elements on either side of the center row prohibits any elevational steering. A 1.75D array can be electronically steered in both azimuth and elevation, but only to a minimal extent as compared to a 2D array. Compared to 1D arrays, both 1.5D and 1.75D arrays are used to control slice thickness through dynamic elevation focusing.
Generally, 1D transducer arrays are optimized for use in two dimensional scanning while 2D transducer arrays are optimized for use in three dimensional scanning. Two dimensional slices of a three dimensional image can be displayed with lower image quality than a two dimensional image obtained from a 1D array. When a user wants to switch between two dimensional imaging and three dimensional imaging, the user must usually change transducer probes. It would be desirable to have a single transducer probe which could be used for both two dimensional and three dimensional imaging and which performs with the image quality of a 1D probe when used for two dimensional imaging.
In accordance with the principles of the present invention, a 2D transducer array is provided which can be used for three dimensional imaging and is switchable to operate as a 1D array for two dimensional imaging. Connections between elements of the 2D array are switched, preferably in the probe itself, so that echo signals are combined prior to being processed by the system beamformer. In an illustrated embodiment the 2D array can be operated with fully populated 1D apertures for two dimensional imaging or with sparsely populated 2D apertures for three dimensional imaging. A transducer probe of the present invention can advantageously be used to periodically acquire a two dimensional image frame using the fully-populated 1D aperture during the acquisition of a three dimensional volume of data by the sparse 2D aperture.